The development of new and improved compounds for use as varistors in electrical surge arresters is a continuing concern in view of the ever increasing demand for electricity and electrically powered devices. Varistors are electrical resistors that do not obey Ohm's law in that the current flowing through a varistor is not proportional to the applied potential voltage. Because of the varistor's non-ohmic behavior, when a line voltage exceeds the breakdown voltage, the surge is carried away through the varistor and the circuit is thereby protected.
Presently, there exist certain compounds made from oxide powders that may optimally be used as varistors. The oxide varistors, in turn, are suitable for use as surge arresters or voltage limiters in electrical devices due to their non-ohmic behavior. Nonohmic behavior is achieved by doping zinc oxide with one or more oxides which results in the formation of voltage barriers at the grain boundaries. The increase in the varistor conductivity is related to temporary breakdown of the grain boundary barriers. Thus, the varistor breakdown voltage (V.sub.b) is inversely related to the average zinc oxide grain size.
Aside from zinc oxide varistors, there are a variety of other known varistors, including silicon carbide, carbon and selenium varistors. However, zinc oxide varistors, which are ceramics that have highly nonlinear electrical conduction characteristics, have several other advantages over the other above-mentioned varistors which are spark gap devices. Accordingly, zinc oxide varistors are especially suitable for use as surge arresters or voltage limiters in electrical systems.
However, despite the suitability of zinc oxide varistors in surge arresters and the like, it is known that the electrical properties and reliability of zinc oxide varistors depend critically on internal homogeneity, i.e., chemical and microstructural. This desired homogeneity is often disrupted by the techniques employed in the preparation of the varistors. For example, the creation of highly sinterable powders and required mixing of dopants into the zinc oxide involves a ball milling technique. However, this required milling technique introduces contaminants into the varistor from the milling medium. Likewise, the fine, highly sinterable powders made by various chemical techniques, such as conventional sol-gel processes, are often difficult to handle and are incapable of filling large dies uniformly prior to the pressing and sintering process.
Known chemical processes for the production of zinc oxide varistors generally involve the use of a hydrous oxide powder which is fabricated by either conventional precipitation techniques or sol-gel techniques. While the sol-gel technique has been demonstrated to be superior to other methods of zinc oxide varistor production, there still are problems associated with its use. Moreover, additional problems are caused by the powders that are used in these techniques in the fabrication of zinc oxide varistors. For example, clumping and agglomeration problems may occur in finely divided zinc oxide powders that are used in small varistor devices. When such clumping of the powder occurs, it becomes difficult to obtain the desired sintered density and grain size in the final product when fabricating a large surge arrester. Because uniform grain size is an essential requirement for the ceramic varistor to properly function as a reliable surge arrester, it is a problem that cannot be ignored. Moreover, these above-mentioned techniques are sensitive to pressure and calcining temperature and cannot tolerate normal day to day variations in conditions which exist in actual manufacturing.
U.S. Pat. No. 4,510,112 discloses a process for producing zinc oxide based varistors that comprises particles of zinc oxide and metal-oxide dopants. Although the process disclosed in the '112 patent promotes densification while restricting liquid formation and grain growth, the problems associated with powder clumping and agglomeration are still present and accordingly, precludes uniform grain size.
Problems similar to those disclosed in the above-mentioned '112 patent result from the sol-gel technique used in the article "Fabrication of High-Field Zinc Oxide Varistors by Sol-Gel Processing", R. J. Lauf and W. D. Bond, Ceramic Bulletin, Vol. 63, No. 2, pp. 278-281 (1984). The powders used in the above-cited article are processed at low temperatures, i.e. 1000.degree. C., to minimize liquid formation during sintering. While the above-mentioned liquid formation was minimized and grain growth was slightly reduced, the resulting preparation still has apparent instability.
Thus, attempts have been made in the field to develop a process for fabricating zinc oxide varistors which have uniform grain size and which can be sintered to full density and which may be used in surge arresters. Hence, the preparation of the oxide powder in the form of gel microspheres and the modified sol-gel process for fabricating same, both of which are developed by this invention are utilized in electrical surge arresters.